### [CVE-2021-43542](https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2021-43542)
![](https://img.shields.io/static/v1?label=Product&message=Firefox%20ESR&color=blue)
![](https://img.shields.io/static/v1?label=Product&message=Firefox&color=blue)
![](https://img.shields.io/static/v1?label=Product&message=Thunderbird&color=blue)
![](https://img.shields.io/static/v1?label=Version&message=unspecified%20&color=brightgreen)
![](https://img.shields.io/static/v1?label=Vulnerability&message=XMLHttpRequest%20error%20codes%20could%20have%20leaked%20the%20existence%20of%20an%20external%20protocol%20handler&color=brightgreen)

### Description

Using XMLHttpRequest, an attacker could have identified installed applications by probing error messages for loading external protocols. This vulnerability affects Thunderbird < 91.4.0, Firefox ESR < 91.4.0, and Firefox < 95.

### POC

#### Reference
- https://www.mozilla.org/security/advisories/mfsa2021-54/

#### Github
No PoCs found on GitHub currently.